The present invention relates to a method for manufacturing a spark plug which has two electrodes (i.e., center electrode and ground electrode) opposed to form a predetermined gap and a noble metal tip welded to at least one of these electrodes.
FIGS. 4A and 4B show a conventional welding method for a spark plug. First, as shown in FIG. 4A, a tip 103a is inserted into a hole of a guide jig 110. The guide jig 110 is used to locate the tip 103a to a predetermined position on a plug electrode 103 (such as a ground electrode or a center electrode). Then, the guide jig 110 is removed upward to leave the tip 103a standing upright on the electrode 103. Then, electric or resistance welding is performed to fix the tip 103a to the plug electrode 103 under a condition that tip 103a is depressed by a welding electrode 120 (refer to FIG. 4B).
According to this conventional welding method, the guide jig may touch the tip 103a standing upright on the electrode 103 when the guide jig is raised upward. Thus, the tip 103a may move or slide on the electrode.
If the hole of guide jig 110 is enlarged to solve this problem, it will deteriorate the accuracy in positioning the tip 103a on the plug electrode 103. In other words, the distance (i.e., discharge gap) between the electrodes will become inaccurate and accordingly the properties of the spark plug will be worsened.
To solve the above problems, the present invention has an object to provide a manufacturing method for a spark plug capable of accurately positioning a tip on a plug electrode before welding the tip to this electrode.
In order to accomplish the above and other related objects, the present invention provides a first method for manufacturing a spark plug having two electrodes opposed to cause a spark discharge and a noble metal tip welded to at least one of these plug electrodes. A first process of the first manufacturing method is performed to clamp the tip by first and second guides of a guide jig approaching to each other in a direction normal to an axial direction of the tip and to locate the tip to a predetermined position on a plug electrode. A second process of the first manufacturing method, succeeding the first process, is performed to depress the tip by a welding electrode under a condition that the tip is held by the guide jig, in which a thickness of the guide jig is smaller than a clearance between the welding electrode and the plug electrode. A third process of the first manufacturing method, succeeding the second process, is performed to remove the first and second guides of the guide jig from the tip in the direction normal to the axial direction of the tip. A fourth process of the first manufacturing method, succeeding the third process, is performed to weld the tip to the plug electrode by supplying electric power to the welding electrode.
According to the first manufacturing method, the first and second guides of the guide jig shift in the direction normal to the axial direction of the tip. This movement surely prevents the guide jig (i.e., either the first guide or the second guide) from touching and moving the tip when the guide jig departs from the tip.
Thus, the first manufacturing method assures an excellent welding operation performed under the condition that the tip is accurately positioned on the plug electrode. The first manufacturing method makes it possible to reduce a manufacturing error of the discharge gap formed between the opposed electrodes.
Furthermore, according to the first manufacturing method, the first and second guides of the guide jig depart from the tip under the condition that a pressing force of the welding electrode is applied on the tip. This surely prevents the tip from being moved by the guide jig (i.e., either the first guide or the second guide). The manufacturing error of the discharge gap can be further reduced.
The present invention provides a second method for manufacturing a spark plug having two electrodes opposed to cause a spark discharge and a noble metal tip welded to at least one of these plug electrodes. A first process of the second manufacturing method is performed to clamp the tip by first and second guides of a guide jig approaching to each other in a direction normal to an axial direction of the tip and to locate the tip to a predetermined position on a plug electrode. A second process of the second manufacturing method, succeeding the first process, is performed to remove the first and second guides of the guide jig from the tip in the direction normal to the axial direction of the tip. A third process of the second manufacturing method, succeeding the second process, is performed to depress the tip by a welding electrode. A fourth process of the second manufacturing method, succeeding the third process, is performed to weld the tip to the plug electrode by supplying electric power to the welding electrode.
According to the second manufacturing method, the first and second guides of the guide jig shift in the direction normal to the axial direction of the tip. This movement surely prevents the guide jig (i.e., either the first guide or the second guide) from touching and moving the tip when the guide jig departs from the tip.
Thus, the second manufacturing method assures an excellent welding operation performed under the condition that the tip is accurately positioned on the plug electrode. The second manufacturing method makes it possible to reduce a manufacturing error of the discharge gap formed between the opposed electrodes.
According to a preferable embodiment of the second manufacturing method, a height of the guide jig is lower than a height of the tip positioned on the plug electrode, when the guide jig is placed on the plug electrode to clamp the tip.
It is also preferable that at least one of the first and second guides of the guide jig has a triangular or comparable notch for guiding the tip. This makes it possible to automatically center the tip to a predetermined position on the plug electrode when the guide jig clamps the tip.